Potential and Challenges of Carbon Sequestration in Urban Soils.

Urban ecosystems comprise of built up lands to provide infrastructure for human habitat and logistics. By 2030, more than 60% of the estimated world population of 8.1 billion will live in urban centers, which are now expanding at ~3 million hectare (Mha) per annum. Urban population in the U.S. increased by 12.1% between 2000-2010, and now accounts for 80.7% of the U.S. population. There are two categories for urban lands: i) urban areas with a population of 50,000 or more, and ii) urban clusters of between ~2,500-50,000 population. There are 486 urban areas and 3,087 urban clusters within the U.S.; and ecosystem characteristics differ among them. Soils of urban ecosystems, constructed by humans, are drastically disturbed, intensively managed, and may be termed as “Anthrosols.” These are managed ecosystems with unique landscape characteristics, pedospheric processes, and biogeochemical cycles. Whereas there is a large potential of carbon (C) sequestration, scientific information on C cycling, interactive pedospheric processes, and soil/ecosystem C budget is scanty. The attainable C sink capacity depends on soil properties (i.e., depth) and management. The strategy is to simultaneously enhance ecological functions and ecosystem services of interest to humans. In this context, increasing biodiversity of plant species (e.g., deep rooted plants) is important. Soils of urban ecosystems are source of several greenhouse gases including N₂O and CO₂. Anaerobic environments and compacted soils of low porosity can also lead to methanogenesis and emission of CH₄. Net soil C sequestration rate, while accounting for the hidden C cost of management-related inputs (e.g., mowing, fertilizer, pesticides, irrigation), must be computed by conducting lifecycle analysis. The gross rate of soil C sequestration may be ~1 Mg ha^-1yr^-1. Management options to improve soil and ecosystem C pool include choice of grass species, recycling of clippings, use of balanced fertilizers, and judicious input of irrigation.